In the past, various different techniques are believed to have been utilized in an attempt to flow air through a contained space of a system including an apparatus for conditioning the temperature of the air. The pressure of such air is related to the flow restrictions associated with such system. Both the speed and torque of an electric motor driving a fan or blower to effect air flow through the system are affected by the flow restrictions in the system. The pressure of air through the apparatus also affects the motor speed and torque. Constant pressure at the output plenum of a zone control type heating or air conditioning system is desirable because it eliminates interaction between zones when dampers are adjusted. One approach of the past involved the rather laborious and time consuming matching of motor speed and torque with various flow restrictions to approximate the desired air pressure for the particular contained space of the particular apparatus or system in which such apparatus was employed. However, this did not accommodate unanticipated variations in the flow restrictions in the system caused by alterations in the system such as malfunctioning dampers or blocked vents. Another approach accomplishes constant pressure in the output plenum by measuring the plenum pressure with a transducer and adjusting the blower speed with a feedback loop to maintain desired pressure. However, this pressure transducer typically comprises a large proportion of the material cost of the control system.
If the fan or blower utilized in the past was of the fan or blade type, an increase in the flow restrictions acting on such fan caused an increase in the speed of the fan and the electric motor driving it and a concurrent pressure increase. Conversely, if the flow restrictions acting on the fan are decreased, the speed of the fan and electric motor and the pressure also decrease. Thus, the speed of the fans and electric motors, and consequently the pressure, varied inversely with a variation of the flow restrictions in the system.
As recognized in coassigned U.S. Pat. No. 4,806,833, incorporated herein by reference in its entirety, a decrease in the static pressure acting on a squirrel cage blower results in a decrease in the speed of the squirrel cage blower and the electric motor driving it. Conversely, if the static pressure acting on the squirrel cage blower is increased, the speed of the squirrel cage blower and its driving electric motor is increased. Thus, it was found that the speed of the squirrel cage blower and its driving electric motor varies directly with a variation in the static pressure. Accordingly, U.S. Pat. No. 4,806,833 discloses a method of operating a system for conditioning air including a variable speed blower for flowing the conditioned air through a contained space having a static pressure therein. In the system of this patent, the speed of the blower is set to provide a preselected flow rate at an existing static pressure in the contained space and the speed of the blower is altered only in response to a variation in the static pressure and only in following relation with the static pressure variation. The speed alteration of the blower is sensed, and the speed of the blower is altered in following relation with the sensed speed alteration to establish the preselected flow rate through the contained space at the varied static pressure acting on the blower.
Although this approach is an improvement over the prior art, it is only a rough approximation of the characteristics needed to achieve constant air flow with respect to changes in static pressures. In general, the systems of the prior art worked reasonably well over a limited range of air flows and static pressures by employing a single slope speed compensation technique. Coassigned Ser. No. 07/385,664, filed July 26, 1989, incorporated herein by reference, recognizes that a much more accurate approach to providing constant flow rate can be implemented in an air handling system by using a microprocessor controlled by a speed compensated torque algorithm. Such speed compensated torque allows the blower motor to maintain air flow in the system independent of variations in the static pressure in the duct work. However, constant flow rate is inapplicable to systems which require constant pressure in response to varying flow restrictions and which are not concerned with varying flow rates.